makefiles/kconfig: Include configuration symbols
makefiles/kconfig: Include configuration symbols

If the generated configuration file is present include it. That way one
can check if certain symbols are being configured using Kconfig.

kaspar030/RIOT,josephnoir/RIOT,OTAkeys/RIOT,x3ro/RIOT,yogo1212/RIOT,toonst/RIOT,mtausig/RIOT,yogo1212/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,basilfx/RIOT,x3ro/RIOT,yogo1212/RIOT,authmillenon/RIOT,miri64/RIOT,basilfx/RIOT,basilfx/RIOT,kYc0o/RIOT,x3ro/RIOT,OlegHahm/RIOT,ant9000/RIOT,josephnoir/RIOT,ant9000/RIOT,mtausig/RIOT,josephnoir/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,x3ro/RIOT,josephnoir/RIOT,toonst/RIOT,smlng/RIOT,smlng/RIOT,kaspar030/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,basilfx/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,toonst/RIOT,kYc0o/RIOT,basilfx/RIOT,OlegHahm/RIOT,miri64/RIOT,yogo1212/RIOT,OlegHahm/RIOT,miri64/RIOT,yogo1212/RIOT,smlng/RIOT,smlng/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,josephnoir/RIOT,jasonatran/RIOT,yogo1212/RIOT,x3ro/RIOT,RIOT-OS/RIOT,toonst/RIOT,mtausig/RIOT,authmillenon/RIOT,mtausig/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,mtausig/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,toonst/RIOT,RIOT-OS/RIOT,smlng/RIOT